[PATCH v3] rust: add global lock support

[Alice Ryhl]

Add support for creating global variables that are wrapped in a mutex or
spinlock. Optionally, the macro can generate a special LockedBy type
that does not require a runtime check.

The implementation here is intended to replace the global mutex
workaround found in the Rust Binder RFC [1]. In both cases, the global
lock must be initialized before first use. The macro is unsafe to use
for the same reason.

The separate initialization step is required because bindgen refuses to
expose __ARCH_SPIN_LOCK_UNLOCKED to Rust as a compile-time constant. It
just generates an `extern "C"` global reference instead. In the future,
we could expose the value of __ARCH_SPIN_LOCK_UNLOCKED to Rust in a way
that Rust can understand. This would remove the need for a separate
initialization step.

Link: https://lore.kernel.org/rust-for-linux/20231101-rust-binder-v1-2-08ba9197f637@xxxxxxxxxx/#Z31drivers:android:context.rs [1]
Signed-off-by: Alice Ryhl <aliceryhl@xxxxxxxxxx>
---
I've been having some trouble with the kunit tests. The import of
__static_lock_ty_$name fails when I try with kunit, but copying the same
code into a sample does not reproduce the error. Suggestions would be
appreciated.
---
Changes in v3:
- Rewrite to provide a macro instead.
- Link to v2: https://lore.kernel.org/r/20240827-static-mutex-v2-1-17fc32b20332@xxxxxxxxxx

Changes in v2:
- Require `self: Pin<&Self>` and recommend `Pin::static_ref`.
- Other doc improvements including new example.
- Link to v1: https://lore.kernel.org/r/20240826-static-mutex-v1-1-a14ee71561f3@xxxxxxxxxx
---
 rust/kernel/sync.rs             |   1 +
 rust/kernel/sync/lock.rs        |  31 +++++-
 rust/kernel/sync/lock/global.rs | 237 ++++++++++++++++++++++++++++++++++++++++
 3 files changed, 268 insertions(+), 1 deletion(-)

diff --git a/rust/kernel/sync.rs b/rust/kernel/sync.rs
index 0ab20975a3b5..2e97e22715db 100644
--- a/rust/kernel/sync.rs
+++ b/rust/kernel/sync.rs
@@ -14,6 +14,7 @@
 
 pub use arc::{Arc, ArcBorrow, UniqueArc};
 pub use condvar::{new_condvar, CondVar, CondVarTimeoutResult};
+pub use lock::global::global_lock;
 pub use lock::mutex::{new_mutex, Mutex};
 pub use lock::spinlock::{new_spinlock, SpinLock};
 pub use locked_by::LockedBy;
diff --git a/rust/kernel/sync/lock.rs b/rust/kernel/sync/lock.rs
index f6c34ca4d819..3ae7a278016d 100644
--- a/rust/kernel/sync/lock.rs
+++ b/rust/kernel/sync/lock.rs
@@ -7,12 +7,14 @@
 
 use super::LockClassKey;
 use crate::{init::PinInit, pin_init, str::CStr, types::Opaque, types::ScopeGuard};
-use core::{cell::UnsafeCell, marker::PhantomData, marker::PhantomPinned};
+use core::{cell::UnsafeCell, marker::PhantomData, marker::PhantomPinned, pin::Pin};
 use macros::pin_data;
 
 pub mod mutex;
 pub mod spinlock;
 
+pub(super) mod global;
+
 /// The "backend" of a lock.
 ///
 /// It is the actual implementation of the lock, without the need to repeat patterns used in all
@@ -117,6 +119,33 @@ pub fn new(t: T, name: &'static CStr, key: &'static LockClassKey) -> impl PinIni
             }),
         })
     }
+
+    /// # Safety
+    ///
+    /// Before any other method on this lock is called, `global_lock_helper_init` must be called.
+    #[doc(hidden)]
+    pub const unsafe fn global_lock_helper_new(state: Opaque<B::State>, data: T) -> Self {
+        Self {
+            state,
+            data: UnsafeCell::new(data),
+            _pin: PhantomPinned,
+        }
+    }
+
+    /// # Safety
+    ///
+    /// * This lock must have been created using `global_lock_helper_new`.
+    /// * Must only be called once for each lock.
+    #[doc(hidden)]
+    pub unsafe fn global_lock_helper_init(
+        self: Pin<&Self>,
+        name: &'static CStr,
+        key: &'static LockClassKey,
+    ) {
+        // SAFETY: The pointer to `state` is valid for the duration of this call, and both `name`
+        // and `key` are valid indefinitely.
+        unsafe { B::init(self.state.get(), name.as_char_ptr(), key.as_ptr()) }
+    }
 }
 
 impl<T: ?Sized, B: Backend> Lock<T, B> {
diff --git a/rust/kernel/sync/lock/global.rs b/rust/kernel/sync/lock/global.rs
new file mode 100644
index 000000000000..c1eb25d37abd
--- /dev/null
+++ b/rust/kernel/sync/lock/global.rs
@@ -0,0 +1,237 @@
+// SPDX-License-Identifier: GPL-2.0
+
+// Copyright (C) 2024 Google LLC.
+
+//! Support for defining statics containing locks.
+
+/// Defines a global lock.
+///
+/// Supports the following options:
+///
+/// * `value` specifies the initial value in the global lock.
+/// * `wrapper` specifies the name of the wrapper struct.
+/// * `guard` specifies the name of the guard type.
+/// * `locked_by` specifies the name of the `LockedBy` type.
+///
+/// # Examples
+///
+/// A global counter.
+///
+/// ```
+/// kernel::sync::global_lock! {
+///     // SAFETY: Initialized in module initializer before first use.
+///     static MY_COUNTER: Mutex<u32> = unsafe { uninit };
+///     value: 0;
+/// }
+///
+/// fn increment_counter() -> u32 {
+///     let mut guard = MY_COUNTER.lock();
+///     *guard += 1;
+///     *guard
+/// }
+///
+/// impl kernel::Module for MyModule {
+///     fn init(_module: &'static ThisModule) -> Result<Self> {
+///         // SAFETY: called exactly once
+///         unsafe { MY_COUNTER.init() };
+///
+///         Ok(MyModule {})
+///     }
+/// }
+/// # struct MyModule {}
+/// ```
+///
+/// A global mutex used to protect all instances of a given struct.
+///
+/// ```
+/// kernel::sync::global_lock! {
+///     // SAFETY: Initialized in module initializer before first use.
+///     static MY_MUTEX: Mutex<()> = unsafe { uninit };
+///     value: ();
+///     guard: MyGuard;
+///     locked_by: LockedByMyMutex;
+/// }
+///
+/// /// All instances of this struct are protected by `MY_MUTEX`.
+/// struct MyStruct {
+///     my_counter: LockedByMyMutex<u32>,
+/// }
+///
+/// impl MyStruct {
+///     /// Increment the counter in this instance.
+///     ///
+///     /// The caller must hold the `MY_MUTEX` mutex.
+///     fn increment(&self, guard: &mut MyGuard) -> u32 {
+///         let my_counter = self.my_counter.as_mut(guard);
+///         *my_counter += 1;
+///         *my_counter
+///     }
+/// }
+///
+/// impl kernel::Module for MyModule {
+///     fn init(_module: &'static ThisModule) -> Result<Self> {
+///         // SAFETY: called exactly once
+///         unsafe { MY_MUTEX.init() };
+///
+///         Ok(MyModule {})
+///     }
+/// }
+/// # struct MyModule {}
+/// ```
+#[macro_export]
+macro_rules! global_lock {
+    {
+        $(#[$meta:meta])* $pub:vis static $name:ident: $kind:ident<$valuety:ty> = unsafe { uninit };
+        value: $value:expr;
+        wrapper: $wrapper:ident;
+        $( name: $lname:literal; )?
+        $(
+            guard: $guard:ident;
+            locked_by: $locked_by:ident;
+        )?
+    } => {
+        $crate::macros::paste! {
+            type [< __static_lock_ty_ $name >] = $valuety;
+
+            #[allow(unused_pub)]
+            mod [< __static_lock_mod_ $name >] {
+                use super::[< __static_lock_ty_ $name >] as Val;
+                type Backend = $crate::global_lock_inner!(backend $kind);
+                type GuardTyp = $crate::global_lock_inner!(guard $kind, Val $(, $guard)?);
+
+                /// # Safety
+                ///
+                /// Must be used to initialize `super::$name`.
+                pub(super) const unsafe fn new() -> $wrapper {
+                    let state = $crate::types::Opaque::uninit();
+                    $wrapper {
+                        // SAFETY: The user of this macro promises to call `init` before calling
+                        // `lock`.
+                        inner: unsafe {
+                            $crate::sync::lock::Lock::global_lock_helper_new(state, $value)
+                        }
+                    }
+                }
+
+                /// Wrapper type for a global lock.
+                pub struct $wrapper {
+                    inner: $crate::sync::lock::Lock<Val, Backend>,
+                }
+
+                impl $wrapper {
+                    /// Initialize the global lock.
+                    ///
+                    /// # Safety
+                    ///
+                    /// This method must not be called more than once.
+                    pub unsafe fn init(&'static self) {
+                        // SAFETY:
+                        // * This type can only be created by `new`.
+                        // * Caller promises to not call this method more than once.
+                        unsafe {
+                            $crate::sync::lock::Lock::global_lock_helper_init(
+                                ::core::pin::Pin::static_ref(&self.inner),
+                                $crate::optional_name!($($lname)?),
+                                $crate::static_lock_class!(),
+                            );
+                        }
+                    }
+
+                    /// Lock this global lock.
+                    pub fn lock(&'static self) -> GuardTyp {
+                        $crate::global_lock_inner!(new_guard $($guard)? {
+                            self.inner.lock()
+                        })
+                    }
+                }
+
+                $(
+                pub struct $guard($crate::sync::lock::Guard<'static, Val, Backend>);
+
+                impl ::core::ops::Deref for $guard {
+                    type Target = Val;
+                    fn deref(&self) -> &Val {
+                        &self.0
+                    }
+                }
+
+                impl ::core::ops::DerefMut for $guard {
+                    fn deref_mut(&mut self) -> &mut Val {
+                        &mut self.0
+                    }
+                }
+
+                pub struct $locked_by<T: ?Sized>(::core::cell::UnsafeCell<T>);
+
+                impl<T> $locked_by<T> {
+                    pub fn new(val: T) -> Self {
+                        Self(::core::cell::UnsafeCell::new(val))
+                    }
+                }
+
+                impl<T: ?Sized> $locked_by<T> {
+                    pub fn as_ref<'a>(&'a self, _guard: &'a $guard) -> &'a T {
+                        // SAFETY: The lock is globally unique, so there can only be one guard.
+                        unsafe { &*self.0.get() }
+                    }
+
+                    pub fn as_mut<'a>(&'a self, _guard: &'a mut $guard) -> &'a mut T {
+                        // SAFETY: The lock is globally unique, so there can only be one guard.
+                        unsafe { &mut *self.0.get() }
+                    }
+
+                    pub fn get_mut(&mut self) -> &mut T {
+                        self.0.get_mut()
+                    }
+                }
+                )?
+            }
+
+            use [< __static_lock_mod_ $name >]::$wrapper;
+            $( use [< __static_lock_mod_ $name >]::{$guard, $locked_by}; )?
+
+            $(#[$meta])*
+            $pub static $name: $wrapper = {
+                // SAFETY: We are using this to initialize $name.
+                unsafe { [< __static_lock_mod_ $name >]::new() }
+            };
+        }
+    };
+
+    {
+        $(#[$meta:meta])* $pub:vis static $name:ident: $kind:ident<$valuety:ty> = unsafe { uninit };
+        value: $value:expr;
+        $( name: $lname:literal; )?
+        $(
+            guard: $guard:ident;
+            locked_by: $locked_by:ident;
+        )?
+    } => {
+        $crate::macros::paste! {
+            $crate::global_lock! {
+                $(#[$meta])* $pub static $name: $kind<$valuety> = unsafe { uninit };
+                value: $value;
+                wrapper: [< __static_lock_wrapper_ $name >];
+                $( name: $lname; )?
+                $( guard: $guard; locked_by: $locked_by; )?
+            }
+        }
+    }
+}
+pub use global_lock;
+
+#[doc(hidden)]
+#[macro_export]
+macro_rules! global_lock_inner {
+    (backend Mutex) => { $crate::sync::lock::mutex::MutexBackend };
+    (backend SpinLock) => { $crate::sync::lock::spinlock::SpinLockBackend };
+    (guard Mutex, $val:ty) => {
+        $crate::sync::lock::Guard<'static, $val, $crate::sync::lock::mutex::MutexBackend>
+    };
+    (guard SpinLock, $val:ty) => {
+        $crate::sync::lock::Guard<'static, $val, $crate::sync::lock::spinlock::SpinLockBackend>
+    };
+    (guard $kind:ident, $val:ty, $name:ident) => { $name };
+    (new_guard { $val:expr }) => { $val };
+    (new_guard $name:ident { $val:expr }) => { $name($val) };
+}

---
base-commit: 93dc3be19450447a3a7090bd1dfb9f3daac3e8d2
change-id: 20240826-static-mutex-a4b228e0e6aa

Best regards,
-- 
Alice Ryhl <aliceryhl@xxxxxxxxxx>